The present invention relates generally to contact of, by or between a gas and a liquid and, more particularly, to an apparatus for and method of gas and liquid contact such as adapted for use in liquid redox processes for the removal of sulfur from gases, such as natural gas, for example.
Hydrogen sulfide (H.sub.2 S) is often encountered as a part of or in conjunction with various gas streams, such as in process streams formed or produced in various industrial processes and in natural gas extracted from the ground. In the presence of oxygen, H.sub.2 S can form various sulfates. Such sulfates can act as pollutants (which can contribute to acid rain), and can also themselves be corrosive or otherwise damaging to machinery and equipment, as well as storage and transportation devices such as pipelines, for example, with which such materials and associated process streams may come into contact.
Currently, generally at least about 15 percent of U.S. natural gas production is treated for H.sub.2 S removal. In general, it is common that high or higher quality natural gas reserves are and have to date been preferentially produced. Thus, it is likely that future natural gas production will include a larger proportion of subquality gas, for example, natural gas which contains sulfur in various amounts.
Various processes for the removal of H.sub.2 S from gas streams are know or have previously been proposed. One such process employs a basic agent such as an amine as an absorbent. In practice, such an amine absorbent can be regenerated for reuse as an absorbent through the application of heat, for example, by treatment with steam. A variety of non-regenerative processes using Fe-based solids, or liquid-based processes using caustic triazine, are also widely employed.
A category of processes, termed in the industry as "liquid redox" processes, are frequently used to effect removal of H.sub.2 S from H.sub.2 S-containing natural gas (termed "sour" gas) and other streams. In such processing, the removed sulfur material is commonly converted either to solid elemental sulfur for subsequent sale or to sulfur cake for ultimate disposal. The term "liquid redox" refers to reduction and oxidation processing which is believed to occur in the liquid phase. Generally, in a typical liquid redox process, a reduction-oxidation ("redox") system is used in which sour gas is exposed to a liquid form of sulfide precipitation agent, catalyst material or absorbent (for example, a metal oxide, in which a metal cation changes from a higher valence state to a lower state upon reaction with the H.sub.2 S), and the gas, now with a substantially reduced level of H.sub.2 S (such gas commonly being referred to as "sweet" gas) is piped onward to its intended use. Conventional liquid redox processes typically employ iron in such a reduction-oxidation cycle. In the cycle, the iron is alternately reduced in an absorber yielding elemental sulfur and oxidized in an oxidizer/regenerator yielding water.
After passing through the absorber, at least a portion of the sulfur will have precipitated out of the absorbent solution as elemental sulfur. The precipitation agent/catalyst/absorbent solution is then typically sent to some form of regeneration apparatus, such as an oxidizer, so as to restore the metal cation in the solution to the desired higher valence state, so that the solution upon return to the absorber will again serve to absorb H.sub.2 S from the gas stream.
Liquid redox processes are generally not economical for treatment processing of streams which contain very small amounts of sulfur, e.g., where the H.sub.2 S concentration is less than a few hundred ppm or the total amount of sulfur is less than a few hundred pounds per day.
For economic reasons, regenerative processes employing liquid redox are more attractively applied to streams where the H.sub.2 S concentration is relatively dilute (e.g., no more than a few percent or less) and where the total amount of sulfur removed is above about 50-100 lbs. per day, preferably above about 200-300 lbs. per day, and less than 25 long tons a per day. When conversion to elemental sulfur is desired for environmental or regulatory reasons, such liquid redox processing can be particularly preferred when total sulfur is on the order of 10 tons or less of sulfur per day.
Such liquid redox processes are favored since they operate at ambient temperatures and have high selectivity for H.sub.2 S. While one of the major attributes of liquid redox processes for removing H.sub.2 S from subquality natural gas is the rapid reaction rate of H.sub.2 S with the liquid redox solution and the subsequent precipitation of elemental sulfur, the process can be marred by a tendency of the sulfur to deposit on internal contact surfaces in the absorber, such as walls of the absorber itself as well as internal contact surfaces such as may be formed by contact devices such as static mixers, packing and the like which are commonly housed in and used in association with absorbers to facilitate and enhance contact of the gas and liquid.
The tendency for such sulfur deposition primarily emanates from the fact that there is always a zone of stagnant fluid associated with a fixed surface. That is, a thin "boundary layer" of non-moving fluid is commonly present adjacent to non-moving surfaces. As a result, sulfur precipitation and deposition onto such non-moving surfaces adjacent to a non-moving fluid boundary layer can be essentially continuous and can eventually act to block or clog one or more of the absorber and associated processing equipment and piping, for example. For example, should an absorber clog, the absorber must typically be taken out of service and cleaned, resulting in plant downtime and an economic penalty to the user.
One possible technique that could be used to keep such surfaces free from solid sulfur deposits would be to maintain a large flow of recirculated liquid through the absorber to produce turbulence to act to scrub stagnant zones within the absorber. However, in addition to the uncertainty in achieving uniform turbulence throughout the absorber, maintaining a large circulation of liquid can be quite costly both in terms of the additional amounts of absorbent which must be maintained in a circulation loop as well as the increased energy and equipment costs associated with establishing and maintaining such constant circulation and turbulence.
It would therefore be desirable to provide a method and apparatus for assuring the prevention of buildup of elemental sulfur on non-moving surfaces within an absorber, without having to resort to continuous circulation of absorbent within the absorber.